<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<style>
    .parent{
width:200px;
height:200px;
    }
    .f1{
        float:left;
        background-color: green;
        width:100px;
        height:100px;
    }
    .main {
        height:100px;
        background-color: pink;
    }
</style>
<body>
    <div class="parent">
        <div class="f1"></div>
       <div class="main">121212121122221222222222222222222222222222222222222222222222222222222</div>
    </div>
</body>
</html>